My friend's ex-partner died of water poisoning just before Christmas. I think he had subscribed to the following myths: that water is fantastic and the more the better; and that salt is evil and everyone should avoid it.

He was small, thin, bedbound due to severe ME/CFS, and most likely had Postural Orthostatic Tachycardia Syndrome. No one had even diagnosed the POTS, let alone told him that people with POTS need more salt than usual. He was avoiding salt because he'd picked up vaguely that it was healthier. He hadn't eaten anything the previous day, too ill, and so decided to get his fluids up by drinking some water. I don't know how much he drank, but it doesn't sound like it was all that much. He was dead within two days. If he'd been supplementing properly with salt, I don't think this would have happened. It's the balance of water to salt that causes the damage, due to osmosis moving water around between cells, and the brain swelling.

Ignore anyone telling you to drink ludicrous amounts of water for no good reason, and if you do need to drink a lot of water, make sure you get salt as well. It's worth generally working out how much salt you're getting and how much you personally need.

I have severe ME/CFS and almost certainly have POTS, so I now aim to drink 2.5l of water a day, with half a teaspoon of salt mixed into each 800ml/27oz water bottle (actually, it's half sea salt and half low-sodium salt, so I get potassium too). Some of us do need that. I'm using a water-drinking app to remind me to drink enough. My blood pressure seems to be a bit better on this, more often normal and less often low, but it's hard to tell as the doctor's been changing my meds quite a lot.